1. Field of the Invention
This invention relates generally to a vehicle back-up control system for assisting a vehicle operator in backing up a vehicle-trailer combination and, more particularly, to a vehicle back-up control system that assists a vehicle operator in backing up a vehicle-trailer combination, where the system employs a hitch angle sensor and an active front steering sub-system.
2. Discussion of the Related Art
Backing up a vehicle-trailer combination requires proper actions by steering and/or braking the vehicle to slow down and/or stabilize the vehicle-trailer combination before a jack-knife condition occurs. Particularly, in order to position the trailer toward the target direction, the vehicle operator typically needs to provide counter-steering inputs, which are opposite to normal steering. The jack-knife condition occurs when the vehicle-trailer combination is moving away from its equilibrium position and the system becomes unstable. In other words, the relative angle between the vehicle and the trailer is diverging from the driver's intended target angle, which usually increases when proper steering and/or braking actions are not taken.
U.S. Pat. No. 6,292,094 discloses a system for controlling a backing maneuver of a vehicle-trailer combination, where the vehicle includes operator-actuated front wheel steering and microprocessor-actuated electric motor driven rear-wheel steering that uses the hand-wheel angle for a desired driver command. In this system, the hand-wheel is mechanically coupled to the road wheels through the steering mechanism. When the vehicle operator turns the hand-wheel to provide the command to the controller, the front wheels turn accordingly regardless of the driver's intention. Therefore, the front wheels may turn in-phase with the rear wheels when the driver's intention is to provide counter steering, which would be required to provide out-of-phase steering between the front and rear wheels.
Some state of the art vehicles employ an active front steering (AFS) system, known to those skilled in the art, in combination with a steer-by-wire system where the steering hand-wheel is mechanically decoupled from the vehicle wheels. In other words, the turning of the hand-wheel is electronically detected, where a controller operates the steering gear to turn the front wheels based on the detected signal. The prior art has proposed utilizing a true steer-by-wire mechanism, where the desired front wheel angle is generated based on the driver command provided through the steering hand-wheel. However, this system is limited to a theoretical control feasibility using a steer-by-wire system and instrumented hitch.
Because the known vehicle-trailer back-up control systems only interpret the driver intentions based on the hand-wheel angle, a counter steering command may produce a wrong wheel angle command that is opposite to the driver's intention. The driver should not perform a counter steer if he/she is using this back-up control feature because the counter steer is performed by the AFS. Therefore, perceiving a driver's intention accurately during trailer back-up is important for controlling the backing up motion of a vehicle-trailer combination.